Piston rings as used in engines have hitherto been made by preparing a stack of workpieces in the form of noncircular rings brought together (a piston ring workpiece aggregate or briefly a piston ring aggregate). The piston ring aggregate (in the form of a non-circular cylinder) is machined over its inner and outer peripheral surfaces to impart thereto a given shape. The machined piston ring aggregate is then severed by forming two longitudinal cuts parallel to the axial direction in the cylindrical aggregate. The severed aggregate with a portion removed may have its outer peripheral surface become circular in cross section when its diameter is reduced with the two longitudinal cuts joined together. Certain apparatus for machining the inner and outer peripheral surfaces of a piston ring aggregate or a stack of workpieces in the form of non-circular rings brought together are described in, e. g., patent literature JP, P S54-21691A and JP, P H6-75814B.
The machining apparatus described in JP, P S54-21691 A is designed to produce a stack of piston rings and is described as characterized by including a workpiece support shaft adapted to carry workpieces constituting a piston ring aggregate comprising a plurality of piston ring workpieces brought and held together, a tool support means radially displaceable in accordance with non-circularities of machinable surfaces of the piston rings, and at least two electromechanical stepping feed means that are computer controllable to displace the tool support means radially as desired. In the apparatus so constructed, rotating the workpiece support shaft with the workpieces or piston ring aggregate carried thereon while driving the stepping feed means under computer control is described to provide machining of the workpieces with their inner and outer peripheral surfaces defined by free curves machined simultaneously with precision as sought.
A numerically controlled lathe disclosed in JP, P H6-75814 B includes a cutter mounting on a carriage that is movable by a linear motor towards and away from a workpiece such as to yield a piston ring with a skirt portion, the linear motor being adapted to be driven under numerical control by a computer to have an outer peripheral surface of the skirt portion formed by lathing the workpiece. The lathe has on a guide section for supporting the carriage that reciprocates, a plurality of sets of juxtaposed rotary members and a biasing means that supports one of the sets of rotary members to prevent the carriage from laterally moving so that a reaction force that the workpiece being lathed exerts on the cutting tool may be accepted by both the rotary members and the biasing means. This provision is described to permit the carriage to reciprocate without jolting or shaking, which in turn provides, among others, permitting workpieces to be cut with precision.
The machining apparatus described in JP, P S54-21691 A, however, necessarily entails the piston ring aggregate machined over its inner and outer peripheral surfaces to be subsequently severed by using a separate severing apparatus and is inconvenient as it requires more than one machine, thus rendering the equipment costly and necessitating a wide space for their installation.
The machining apparatus described in JP, P S54-21691 A, especially in connection with FIG. 5, further requires that the tool support means having a cutting tool for machining the outside of a workpiece be mounted on a sliding head of which a movement is controlled by a pair of step feed means that are arranged side by side lengthwise of it. The sliding head has on it a second sliding head that is capable of movement by the step feed means in the same direction as that in which the first sliding head is movable. And, a tool support means having a cutting tool for machining the inside of the workpiece is required to be mounted on the second sliding head. The apparatus so constructed has, of necessity, drive systems interlaced and complicated and, also complicating control units therefor, inconveniently renders the entire equipment costly.
The NC lathe described in JP, P H6-75814 B tailored to machine outer peripheral surfaces is not adapted to simultaneously machine both inside and outside of a workpiece and thus is unsatisfactory in productivity. The requirement for the inside and outside of a workpiece to be separately shaped adversely affects their machined concentricity and here again leaves much to be desired in achievable machining accuracy.
Also, a piston ring aggregate machined over its inner and outer peripheral surfaces must, here too, be subsequently severed by using a separate severing apparatus and inconveniently requiring more than one machines renders the equipment costly and necessitates a wide space for their installation.
It is accordingly an object of the present invention to eliminate these disadvantages met in the prior art and to provide a combined machining apparatus that can perform both the peripheral (inner and outer) machining and severing of piston ring workpieces to prepare the piston rings, the apparatus having an increased productivity and requiring less equipment cost and less space for its installation.